1. Field of the Invention
This invention relates to a device for opening a circuit, which is connected to a protective circuit comprising a constant voltage element disposed between two lines. More specifically, when an overvoltage greater than a rated voltage of the constant voltage element is continuously applied between the two lines, the device for opening the circuit opens the connection between the two lines preventing the overvoltage from being applied to a load side. Further, this invention provides an device for protecting the circuit from overvoltage, in which the device for opening the circuit and the constant voltage element are formed integrally.
2. Description of the Prior Art
In FIG. 34 there is shown a conventional protective circuit (a), in which a constant voltage element (c) such as a silicone surge absorber is disposed between two power source lines or communication lines. The protective circuit is disposed in front of an electronic circuit in an electronic apparatus in order to protect it from overvoltage such as surge or the like. As shown in FIG. 34, protective circuit (a) comprises constant voltage element (c) disposed on a line between two lines A, A' connected to an electronic circuit (b), a fuse (d) for separating the circuit connected in series to constant voltage element (c) and a line intercepting fuse (e) connected in series to line A.
When a surge is applied instantaneously to lines A, A' through an input side, it can be absorbed by constant voltage element (c). Further, when the electronic apparatus is connected by mistake to a power source whose voltage is greater than the rated voltage, or a communication line is connected by mistake to the power source, or the overvoltage greater than rated voltage of constant voltage element (c) is applied;: continuously to lines A, A', fuse (d) is fused to prevent a short-cut destruction or burning of constant voltage element (c), thereby constant voltage element (c) is separated from line A'. Further, after constant voltage element (c) has been separated therefrom, there is the possibility that electronic circuit (b) may be destroyed since overvoltage is applied to it continuously. Therefore, fuse (d) is fused to intercept line A, so that protective circuit (a) itself is opened.
In order to protect electronic circuit (b) from overvoltage, fuse (d) as well as fuse (e) must be fused certainly. A fusing property of a fuse depends greatly on a current amount, a current applying time or the like. Accordingly, there occurs a slight time gap between the time when fuse (d) fuses and the time when fuse (e) fuses. During the foregoing slight time gap, there is a danger that the overvoltage applied to electronic circuit (b) may destroy it.